This invention relates to acoustic ink printing and, more particularly to, a method and apparatus which allows for cleaning and maintaining AIP printheads which implement unique orifice plates, and where a wetted wiping element is cleaned to allow for re-use of the wiping element.
It has been shown that acoustic ink printers which have printheads with emitters, including acoustically illuminated spherical or Fresnel focusing lenses can print precisely positioned picture elements (pixels) at resolutions that are sufficient for high quality printing of complex images. Significant effort has gone into developing acoustic ink printing, see for example, U.S. Pat. Nos. 4,308,547; 4,751,530; 4,697,195; 4,751,530; 4,751,534; 5,028,937; and 5,041,849, all of which are among many commonly assigned to the present assignee.
For performing acoustic printing, each of the emitters of the printhead launches a converging acoustic beam into a pool of ink, with the angular convergence of the beam being selected so that it comes to focus at or near the free surface (i.e., the liquid/air interface) of the pool. Moreover, controls are provided for modulating the radiation pressure which each beam exerts against the free surface of the ink. That permits the radiation pressure from each beam to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of surface tension, whereby individual droplets of ink are emitted from the free surface of the ink on command, with sufficient velocity to deposit them on a nearby recording medium.
A main attraction of acoustic ink printing is the ability to control droplet size based on the frequency of the signal provided, rather than providing on the size of the nozzle emitting the droplet. For example, an AIP printer may emit droplets magnitude in size smaller than the AIP openings. On the other hand, conventional ink jet printing requires a minimization of the nozzle itself to obtain small droplets.
While this is a benefit of AIP type printing, the size of the droplet ejectors used in acoustic ink printing are nevertheless very small. In consideration of this, maintaining the droplet ejectors in a clean state is an extremely important aspect of proper operation. Not only can dirt particles and dust (particularly paper dust) clog the ejector ports, but ejected ink droplets which do not adhere to the recording medium or have such low velocity that they return back to the orifice plate, and can build up enough to disrupt the printing process. Additionally, whereas many conventional ink jet printers require the replacement of the printheads after a somewhat short period of time, AIP printheads can have an indefinite life span. As part of extending this useful life, maintaining the printheads clean is an important aspect.
Existing examples of printhead cleaning are substantially directed to cleaning of printheads configured to use nozzles, whereas acoustic printheads are nozzleless. For nozzle type printheads, a wiper blade is a common device used for cleaning.
However, an ink jet printhead configuration is significantly different from the printhead of an acoustic ink printer. Therefore, attempting to use a wiper blade cleaning device or other cleaning method or apparatus designed for nozzle type printheads will not achieve desired results. For example, use of a wiper blade cleaning device with acoustic ink printheads may result in clogging of the printhead rather than accomplishing the desired cleaning.
It has also been suggested that a non-wiping technique for improving the cleanliness of exposed surfaces of droplet ejectors for a fixed printhead could be used. However, while such a technique has benefits, it is less desirable for moving printheads and also involves significant engineering considerations and is more specifically designed to a fixed printhead situation.
U.S. patent application Ser. No, 09/340,741 entitled METHOD AND APPARATUS FOR CLEANING/MAINTAINING OF AN AIP TYPE PRINTHEAD, filed Jun. 28, 1999 and assigned to the same assignee, describes an apparatus and method of cleaning AIP type printheads. However, the described device only allows a single use of a portion of a cleaning element, such that the cleaning element becomes exhausted.
It has been determined desirable to find a method and apparatus of cleaning/maintaining acoustic ink printheads which have unique orifice plate design in which the ink menisci are maintained at an entrance edge of the orifice plate, defined by a very thin lip. It is also desired that such a method and apparatus be able to clean a cleaning element of the apparatus such that the cleaning element may be re-used.
In accordance with the present invention, there is provided a method and apparatus for providing in combination or individually a flooding, dry and wet wiping of acoustic ink printheads for maintaining the cleanliness of the exposed surfaces of the printhead. A flooding procedure initially attempts to use the ink of the printhead in the cleaning process. Following the flooding operation ink on the outside surface of the orifice plate is removed by use of wiping over it with a compliant wiper blade. Next, ink inside the orifice bore is removed using a self-cleaning wet wiping station. The wiping station of the present invention consists of a wiping element designed with an absorbent, hydrophilic, compliant material, a washing fluid which wetted the wiping element, and a squeegee which removes excess fluid from the wiping element prior to the cleaning process. The washing fluid and squeegee being further used to clean the wiping element following cleaning of the printhead.
During a first step of the wiping station operation, the wetted wiping element is pushed over the orifices while the printhead and wiping element are moved in opposite directions. The dirty ink, because of a higher pressure inside the printhead, is unable to reenter the printhead and is absorbed by the wiping element. In a following step, pressure inside the printhead is decreased to enable the menisci to retreat inside the lip. When the orifice is again wiped with the wiping element, the remaining ink is removed from the bore of each orifice as well as from the exit surface of the submerged lip, due to absorption into the absorbent material. Subsequent to the second step, the wiping element is passed through wash fluid and squeegeed a number of times until the dirty ink is removed from the wiping element. This procedure cleans the wiping element so that on a subsequent cleaning of the printhead, the wiping element can be reused.